1. Field of the Invention
The present invention relates to thermal imaging apparatus, and more particularly to thermal imaging apparatus employing non-linear scanning.
2. Description of Related Art
Various types of thermal imaging devices are known in the art. These include staring arrays, parallel scan devices and serial scan devices. Serial scan devices which employ a plurality of detectors arranged in a linear array and interconnected to provide time delay and integration are described in Israel Patent 39,389 issued to Laakmann.
The article "An Integrating Detector for Serial Scan Thermal Imaging," by C. T. Elliott, et al., Infrared Physics, Vol. 22, pages 31-42, 1982 describes the use of a Mercury Cadmium Telluride "SPRITE" detector for thermal imaging. The SPRITE detector is operative to perform time delay and integration within the detector material itself as controlled by the bias voltage applied across the two contacts of the SPRITE detector.
The SPRITE detector itself is described in U.K. Patents 2119508, published Oct. 26, 1983; 2127619, published Apr. 11, 1984; and U.S. Pat. Nos. 4,572,953, 4,679,063, and 4,691,107.
Many disadvantages exist when using a SPRITE in a thermal scanning device. Disadvantages include the fact that the SPRITE requires a constant current source; the SPRITE requires a constant scan speed; and to maintain optimum performance, the constant current source and scan speed must be closely matched. With these limitations, imaging apparatus using non-linear scanning systems have been impossible because the time delay and integration output of the SPRITE detector will not correspond to the changing speed of the scanning device. These disadvantages were overcome by a thermal imaging apparatus employing a SPRITE detector as described and claimed in U.S. Pat. No. 5,289,006 to Gal, which is incorporated by reference herein.
However, other disadvantages exist with such a thermal imaging device. The applied bias voltage to the detector is a function of both the detector and the scan speed, but one disadvantage is that the bias voltage may not be correctly aligned to the scanner velocity as a result of a phase lag caused by the scanner sensor, associated electronics and the detector time constant. The applied bias mis-alignment with the scanner velocity translates into a loss of output signal because of a reduction in the Modulation Transfer Frequency parameter of the SPRITE detector and a degradation in the signal-to-noise integration efficiency. The signal output of the SPRITE detector consists of the applied bias voltage at a reduced magnitude and the signal component resulting from the photon flux. Compensating for the detector rise time enables for the removal of the applied bias component from the detector output signal which if not removed may result in a signal where the photonic signal is much smaller than the applied bias voltage.